Over the years, numerous simplified test systems have been designed to rapidly detect the presence of a target ligand of interest in biological, environmental and industrial fluids. In one of their simplest forms, these assay systems and devices usually involve the combination of a test reagent which is capable of reacting with the target ligand to give a visual response and an absorbent paper or membrane through which the test reagents flow. Paper products, glass fibers and nylon are commonly used for the absorbent materials of the devices. In certain cases, the portion of the absorbent member containing the test reagents is brought into contact, either physically or through capillarity, with the sample containing the target ligand. The contact may be accomplished in a variety of ways. Most commonly, an aqueous sample is allowed to traverse a porous or absorbent member, such as porous polyethylene or polypropylene or membranes by capillarity through the portion of the porous or absorbent member containing the test reagents. In other cases, the test reagents are pre-mixed outside the test device and then added to the absorbent member of the device to ultimately generate a signal.
Commercially available diagnostic products employ a concentrating zone methodology. In these products, such as ICON.RTM. (Hybritech Incorporated), TESTPACK.TM. (Abbott Laboratories) or ACCULEVEL.RTM. (Syva Corporation), the device contains an immunosorbing or capture zone within a porous member to which a member of a specific binding pair is immobilized. The surface of the porous member also may be treated to contain one or more elements of a signal development system. In these devices, there is a liquid absorbing zone which serves to draw liquid through the immunosorbing zone, to absorb liquid sample and reagents and to control the rate at which the liquid is drawn through the immunosorbing zone. The liquid absorbing zone is either an additional volume of the porous member outside of the immunosorbing zone or an absorbent material in capillary communication with the immunosorbing zone. Many commercially available devices and assay systems also involve a wash step in which the immunosorbing zone is washed free of non-specifically bound signal generator so that the presence or amount of target ligand in the sample can be determined by examining the porous member for a signal at the appropriate zone.
The devices described herein do not use bibulous or porous materials, such as membranes and the like as substrates for the immobilization of reagents or to control the flow of the reagents through the device. A disadvantage of, for example, membranes in diagnostic devices is that on both microscopic and macroscopic scales the production of membranes is not easily reproducible. This can result in diagnostic devices which have differential properties of non-specific binding and flow characteristics. The time gates of this invention can, however, be embedded in membranes or used in devices with membranes. Membranes are very susceptible to non-specific binding which can raise the sensitivity limit of the assay. In the case of immunochromatographic assay formats such as those described in U.S. Pat. Nos. 4,879,215, 4,945,205 and 4,960,691, the use of membranes as the diagnostic element requires an even flow of reagents through the membrane. The problem of uneven flow of assay reagents in immunochromatographic assays has been addressed in U.S. Pat. Nos. 4,756,828, 4,757,004 and 4,883,688, incorporated herein by reference. These patents teach that modifying the longitudinal edge of the bibulous material controls the shape of the advancing front. The devices of the current invention circumvent these membrane associated problems by the use of defined surfaces, including grooved surfaces, capillarity, time gates, novel capillary means, including channels and novel fluid flow control means alone or in various combinations, all of which are constructed from non-absorbent materials. In a preferred mode of this invention, the capillary channel of the diagnostic element is composed of grooves which are perpendicular to the flow of the assay reagents. The manufacture of grooved surfaces can be accomplished by injection molding and can be sufficiently reproducible to provide control of the flow of reagents through the device.
In addition to the limitations of the assay devices and systems of the prior art, including the limitations of using absorbent membranes as carriers for sample and reagents, assay devices generally involve numerous steps, including critical pipetting steps which must be performed by relatively skilled users in laboratory settings. Accordingly, there is a need for one step assay devices and systems, which, in addition to controlling the flow of reagents in the device, control the timing of the flow of reagents at specific areas in the device. In addition, there is a need for assay devices which do not require critical pipetting steps but still perform semi-quantitative and quantitative determinations. The inventive devices and methods of this invention satisfy these needs and others by introducing devices which do not require precise pipetting of sample, which do not use absorbent members, which include novel elements called time gates for the controlled movement of reagents in the device and which are capable of providing quantitative assays.